This invention relates to the transmission of digital data on wireless local area networks (LANs). More particularly, this invention relates to a means of reducing the DC tracking inherent in a closed-loop VCO based FM transmission system by keeping track of the DC bias of the data signal to be transmitted and inverting selected blocks of data to reduce overall DC bias. The present invention has application in numerous communication systems which rely on the transmission of digital data over a wireless channel.
Transmitters in digital communication systems use modulators to process data signals prior to transmission of the signals over a wireless channel. The processed data signals are called modulated signals. Similarly, remote receivers in digital communication systems use demodulators to recover the data signals from the received modulated signals.
A commonly used modulation technique is frequency modulation (FM) for which a modulator comprises a voltage controlled oscillator (VCO). The VCO converts an input voltage (data signal) into an output frequency (modulated signal). When the input voltage to the VCO rises, the output frequency of the VCO increases a corresponding amount. Similarly, when the input voltage to the VCO falls, the output frequency of the VCO decreases a corresponding amount. Thus, the modulated signal has, in its time variant frequency, the information contained in the varying amplitude of the data signal. The FM modulated signal is transmitted over the wireless channel to the remote receiver.
A frequently used demodulator is an AC coupled FM demodulator. The FM demodulator converts an input frequency (received modulated signal) into an output voltage (received data signal). When the frequency of the receive modulated signal increases, the output voltage of the FM demodulator rises a corresponding amount. Similarly, when the frequency of the received modulated signal decreases, the output voltage of the FM demodulator falls a corresponding amount. Thus, the received data signal has, in its amplitude, the information that is contained in the changing frequency of the received modulated signal.
The Federal Communications Commission (FCC) regulates the bands of frequencies which may be used by transmitters to transmit modulated signals over the channels of a LAN. It is important that the transmitters only transmit frequencies within the limits set by the FCC because failure to do so would cause a corruption of the modulated signals propagating over the channels of the LAN. Typically, a driver circuit is used in the transmitter to band pass filter the modulated signal such that the FCC limits are met.
It has long been the state of the art to employ a closed-loop VCO in the transmitter to ensure that the band pass filter of the driver does not attenuate data signal information. The closed-loop VCO ensures that the frequency of the modulated signal is normally in the center of the band of frequencies defined by the FCC for a particular channel. The closed-loop VCO eliminates drift in the frequency spectrum of the modulated signal due to component tolerances, ambient temperature variation and component aging.
A closed-loop VCO FM modulator employs a phase detector (PD) in a feedback loop from the output of the VCO to an error amplifier in the data signal path. The PD is provided with a reference frequency, Fref, the value of which is set to the mid-frequency value of the channel defined by the FCC. The PD outputs a phase error (Pe) voltage which is proportional to the difference between the output frequency of the VCO (modulated signal) and Pref. The error amplifier generates an error signal (Es) from Pe and the data signal and the Es is input to the VCO. The loop gain introduced by the PD and associated circuitry ensures that the output frequency of the VCO is equal to Pref when the data signal is at a null. Thus, drift in the frequency spectrum of the modulated signal is eliminated and the extremes in the frequency content of the modulated signal due to the data signal do not fall outside the PCC limits for the particular channel.
The benefit of a reduction in frequency drift realized by using a closed-loop VCO FM modulator comes at a cost. The closed-loop VCO is affected by the integrated DC offset (bias) of the data signal because the closed-loop VCO is inherently AC coupled. In an extreme case, for example when the data signal is at a "logic high" for an extended period of time (thereby having a positive average DC offset), the closed-loop VCO will respond by forcing its output frequency to Fref. Thus, the fact that the data signal is equal to a logic high is not recognized by the remote receiver.
A prior art technique commonly used for reduce DC bias tracking is to reduce the loop bandwidth of the closed-loop VCO; however, a slower loop response has the undesired effect of limiting start up speed. Eye diagrams (graphic indicators of bit error rates in the system) still show undesirable closure when bandwidth reduction is employed and thus bit error rates are, unfortunately, still high.
It has long been the state of the art to AC couple a received data signal in receivers which employ FM demodulators. AC coupling in an PM receiver ameliorates the DC bias in the received data signal due to component tolerances, ambient temperature variation and component aging. However, AC coupled FM demodulators experience all of the drawbacks of closed-loop VCO FM modulators discussed hereinabove due to DC bias tracking. Thus there is a long felt need in the art for a closed-loop VCO FM transmitter and AC coupled FM receiver system which achieves desirable frequency characteristics and low bit error rates despite the presence of DC bias in the digital data signals to be modulated.
It is an object of the present invention to produce a system for the transmission of data which utilizes a closed-loop VCO frequency modulator and which enjoys the reduced DC bias of the transmitted data signal.
It is a further object of the present invention to produce a system for the reception of data which utilizes an AC coupled frequency demodulator and enjoys the reduced DC bias of the received data signal.